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Versions: 00                                                            
Network Working Group                                           L. Gong
Internet Draft                                                 W. Cheng
Intended status: Standards Track                           China Mobile
Expires: October 8, 2022                                         C. Lin
                                                                M. Chen
                                                   New H3C Technologies
                                                                                                                            R. Chen
                                                        ZTE Corporation
                                                          April 8, 2022



           Advertising Exclusive Links for Flex-Algorithm in IGP
              draft-gong-lsr-exclusive-link-for-flex-algo-00


Abstract

   This document proposes a method to advertise exclusive links for
   Flex-Algorithm in IGP.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 1, 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors. All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document. Code Components extracted from this
   document must include Simplified BSD License text as described in
   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.

Table of Contents


   1. Introduction ................................................ 2
      1.1. Requirements Language .................................. 2
   2. Problem Statement ........................................... 3
   3. New Extension to Prune Links ................................ 3
   4. Backward Compatibility Method in IS-IS ...................... 5
   5. Security Considerations ..................................... 5
   6. IANA Considerations ......................................... 5
   7. References .................................................. 5
      7.1. Normative References ................................... 5
      7.2. Informative References ................................. 5
   8. Acknowledgments ............................................. 6
   Authors' Addresses ............................................. 7

1. Introduction

   Flexible Algorithm (Flex-Algorithm) allows IGP to compute
   constraint-based paths. [I-D.ietf-lsr-flex-algo] specifies the usage
   of Flex-Algorithm in Segment Routing (SR) data planes - SR MPLS and
   SRv6. [I-D.ietf-lsr-ip-flexalgo] extends the Flex-Algorithm for
   native IPv4 and IPv6 data planes.

   In some scenarios, exclusive links may be deployed for Flex-
   Algorithm, but not for best-effort service. However, these links
   cannot be pruned in normal SPF calculation, and unexpected flows may
   be steered into these links.

   This document proposes a method to advertise exclusive links for
   Flex-Algorithm in IGP.

1.1. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in


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   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2. Problem Statement

   Flex-Algorithm allows IGP to compute the best paths along the
   constrained topology.

   A network topology is shown in Figure 1. Node A, B, C and D have an
   extra link between each other. These links have EAG attribute of
   "red" color.

   Flex-Algorithm 128 are enable on Node A, B, C and D, with metric-
   type of IGP cost and EAG rule of including "red". The topology used
   by Flex-Algorithm 128 is shown in Figure 2.

   Flex-Algorithm 128 are used to transmit particular flows, such as
   network slice. The links used by Flex-Algorithm 128 are sub-
   interfaces with dedicated queues for bandwidth guarantee. So it is
   expected that only the particular flows are transmitted on these
   links using Flex-Algorithm 128. However, these links are also
   contained in the default topology used by normal SPF calculation,
   and unexpected flows of best-effort service may be steered into
   these links. Therefore, it is a problem that exclusive links for
   Flex-Algorithm cannot be pruned in normal SPF calculation.

   A======C------E
   ||     ||     |
   ||     ||     |
   ||     ||     |
   B======D------F

   Figure 1

   A------C
   |      |
   |      |
   |      |
   B------D

   Figure 2

3. New Extension to Prune Links

   A new Link Flags sub-TLV is defined in IS-IS. The format is as the
   following:



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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Flags                             ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o Type: TBD.

   o Length: Variable, dependent on the size of the Flags field. MUST
      be a multiple of 4 octets.

   o Flags: Following flags are currently defined.

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |P|             ~
      +-+-+-+-+-+-+-+-+

        o P-Flag: Pruning the link during SPF calculation.

   The Link Flags sub-TLV is advertised in the TLVs/sub-TLVs below:

   o TLV-22 (Extended IS reachability) [RFC5305]

   o TLV-222 (MT-ISN) [RFC5120]

   o TLV-23 (IS Neighbor Attribute) [RFC5311]

   o TLV-223 (MT IS Neighbor Attribute) [RFC5311]

   The Link Flags sub-TLV with P-Flag can be advertised for the
   exclusive links used by Flex-Algorithm, so that these links will be
   pruned during normal SPF calculation.

   When using the Link Flags sub-TLV, all nodes in the same area or
   level must support this sub-TLV. Otherwise routing loops may be
   caused by topology inconsistences.

   Relevant extensions for OSPF will be included in the future version
   of this draft.







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4. Backward Compatibility Method in IS-IS

   As specified in [RFC5305], if a link is advertised with the maximum
   link metric (2^24 - 1), this link MUST NOT be considered during the
   normal SPF computation in IS-IS.

   Instead of advertising the Link Flags sub-TLV in Section 3, the
   metrics of exclusive links for Flex-Algorithm may be advertised by
   the value of (2^24 - 1). It can also lead to the pruning of these
   links in normal SPF computation.

   If the associated Flex-Algorithm needs to use IGP-Cost as its
   metric-type in path calculation, the Flex-Algorithm-associated
   Generic Metric sub-TLV defined in [I-D.cheng-lsr-flex-algo-metric]
   can be advertised to carry the actual value.

5. Security Considerations

   TBD

6. IANA Considerations

   Link Flags sub-TLV (TBD)

7. References

7.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, May 2017

   [I-D.ietf-lsr-flex-algo] Psenak, P., Hegde, S., Filsfils, C.,
             Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm",
             draft-ietf-lsr-flex-algo-18 (work in progress), October
             2021.

7.2. Informative References

   [I-D.ietf-lsr-ip-flexalgo] Britto, W., Hegde, S., Kaneriya, P.,
             Shetty, R., Bonica, R., and P. Psenak, "IGP Flexible
             Algorithms (Flex- Algorithm) In IP Networks", draft-ietf-
             lsr-ip-flexalgo-04 (work in progress), December 2021.




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   [RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
             Topology (MT) Routing in Intermediate System to
             Intermediate Systems (IS-ISs)", RFC 5120, DOI
             10.17487/RFC5120, February 2008, <https://www.rfc-
             editor.org/info/rfc5120>.

   [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
             Engineering", RFC 5305, DOI 10.17487/RFC5305, October
             2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5311] McPherson, D., Ed., Ginsberg, L., Previdi, S., and M.
             Shand, "Simplified Extension of Link State PDU (LSP) Space
             for IS-IS", RFC 5311, DOI 10.17487/RFC5311, February 2009,
             <https://www.rfc-editor.org/info/rfc5311>.

   [I-D.cheng-lsr-flex-algo-metric] Cheng, W, "Advertisement of
             Dedicated Metric for Flexible Algorithm in IGP", draft-
             cheng-lsr-flex-algo-metric-00 (work in progress), February
             2022.

8. Acknowledgments

   The authors would like to thank the following for their valuable
   contributions of this document:

   TBD






















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Authors' Addresses

   Liyan Gong
   China Mobile

   Email: gongliyan@chinamobile.com


   Weiqiang Cheng
   China Mobile

   Email: chengweiqiang@chinamobile.com


   Changwang Lin
   New H3C Technologies

   Email: linchangwang.04414@h3c.com


   Mengxiao Chen
   New H3C Technologies

   Email: chen.mengxiao@h3c.com


   Ran  Chen
   ZTE Corporation 

   Email: chen.ran@zte.com.cn























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